Make wal_compression PGC_SUSET rather than PGC_USERSET.

When enabling wal_compression, there is a risk to leak data similarly to
the BREACH and CRIME attacks on SSL where the compression ratio of
a full page image gives a hint of what is the existing data of this page.
This vulnerability is quite cumbersome to exploit in practice, but doable.

So this patch makes wal_compression PGC_SUSET in order to prevent
non-superusers from enabling it and exploiting the vulnerability while
DBA thinks the risk very seriously and disables it in postgresql.conf.

Back-patch to 9.5 where wal_compression was introduced.

doc/src/sgml/config.sgml

@@ -2303,6 +2303,7 @@ include_dir 'conf.d'
         <xref linkend="guc-full-page-writes"> is on or during a base backup.
         A compressed page image will be decompressed during WAL replay.
         The default value is <literal>off</>.
+        Only superusers can change this setting.
        </para>
 
        <para>

src/backend/utils/misc/guc.c

@@ -995,7 +995,7 @@ static struct config_bool ConfigureNamesBool[] =
 	},
 
 	{
-		{"wal_compression", PGC_USERSET, WAL_SETTINGS,
+		{"wal_compression", PGC_SUSET, WAL_SETTINGS,
 			gettext_noop("Compresses full-page writes written in WAL file."),
 			NULL
 		},